Asphaltenes are the heaviest and the most polar components of a petroleum cut. A petroleum fluid is in fact a continuous series of hydrocarbons (alkanes, naphthenes, aromatics). Asphaltenes are defined, according to the French AFNOR T 60-115 standard, as the fraction of a petroleum fluid that flocculates (precipitates) in n-heptane and is soluble in benzene. It is therefore a solubility class. These asphaltenes flocculate under the effect of three factors: temperature, pressure or chemical composition variation. FIG. 1 shows the behaviour of an asphaltenic crude under pressure.
This curve shows the amount of asphaltenes in the liquid fluid after a possible flocculation. When the pressure decreases, the amount of asphaltenes falls, which proves that part of the asphaltenes has flocculated. Flocculation is maximum (and the amount of asphaltenes in the liquid minimum) when the bubble-point pressure (appearance of the first vapour bubble) is reached. Below this pressure, the appearance of a vapour phase rich in alkanes and therefore in flocculating products allows the liquid to become asphaltenes-friendly again.
A change in the composition of the fluid can also induce the flocculation phenomenon. Addition of light alkanes produces flocculation of the asphaltenes, whereas addition of aromatics allows to stabilize them in the crude.
During production of a well, the hydrocarbon-containing fluid undergoes pressure, temperature and composition variations. It is therefore likely to flocculate. This flocculation is translated into the deposition of asphaltenes in the pores of the reservoir, which consequently leads to a change in the porosity of the medium, in its permeability and eventually in the hydrocarbons production. In extreme cases, this flocculation can cause clogging of the pores and lead to close the well. It is therefore important to be able to predict this phenomenon.
The following publications are representative of the state of the art:                Hirschberg L., De Jong B., Schipper J., Meijer J., Soc. Pet. Eng. J., June, 283 Paper No.11202, 1984,        Mansoori G., Jiang T., 3rd European Conference on Enhanced Oil Recovery, Rome, April 1985, or        Szewczyk V., Béhar E., Compositional model for predicting asphaltenes flocculation, Fluid Phase Equilibria, 156-160, pp. 459-469, 1999.        
None of the many flocculation models described in the open literature has a predictive character. Basically, all these representations consider a conventional model for representing the liquid-vapour behaviour of the hydrocarbon-containing fluid. Flocculation is described, but there are no means of predicting the flocculation phenomenon. Certain models consider asphaltenes as solid particles suspended in the crude and flocculation is represented by a colloidal model. In another thermodynamic approach, asphaltenes are considered to be a new liquid phase. The appearance of a liquid or solid phase, according to the aforementioned authors, therefore has to be controlled. All these models require experimental data such as those of FIG. 1 for example in order to be used.
In order to account for the pressure and composition variation, two approaches can be considered. The first one consists in having a rather detailed compositional model of the fluid and a flocculation curve. It is then possible to extrapolate the model in the domain close to the calibration domain of the model. The second method consists in determining flocculation curves as a function of the pressure for different fluid compositions so as to be able to generate a family of flocculation curves in the domain of interest. These two approaches appear to be unusable in practice within the context of industrial simulation softwares. The first approach is difficult to implement because a detailed compositional model of the fluid has to be included in these simulators, which is very costly in computing time. The second approach is also too costly, but in experimentation time: acquisition of data relative to flocculation under pressure is long and delicate, and it is not possible to accumulate curves for a large number of fluid compositions.